The present invention is directed to a closure mechanism or fastener for medical implants and more specifically, to a coaxially nested fastener with an up- or down-loaded central set screw.
Bone screws are utilized in many types of spinal surgery, such as for osteosynthesis, in order to secure various implants to vertebrae along the spinal column for the purpose of stabilizing and/or adjusting spinal alignment. Although both closed-ended and open-ended bone screws are known, open-ended screws are particularly well suited for connections to rods and connector arms, because such rods or arms do not need to be passed through a closed bore, but rather can be laid or urged onto an open receiver channel of such a head.
Typical open-ended bone screws include a threaded shank with a pair of parallel projecting branches or arms which form a yoke with a U-shaped slot or channel to receive a rod. Hooks and other types of connectors, as are used in spinal fixation techniques, may also include open ends for receiving rods or portions of other structure.
A rod is positioned in the U-shaped channel in generally perpendicular relation to the shank and the open end of the yoke is closed off by a closure device. The closure device is tightened against the rod to clamp the rod in place against the bottom of the channel. The closure device must positively secure the rod in place to prevent rotational or translational movement of the rod relative to the bone screw and the bone in which it is anchored. Conventional types of closure devices include a threaded fastener which is screwed into threads formed into the surfaces forming the U-shaped channel or a nut with an inner thread that mates with threads disposed on outer surfaces of the arms.
Fasteners disposed within the arms of a bone screw head are often preferable to nuts as such fasteners take up less space. In order to perform adequately, such a fastener is tightly torqued relative to the bone screw to secure the bone screw to the rod and prevent relative rotation or translation. When the rod is straight, this is readily accomplished. However, in typical spinal fixation applications, the rod is almost always bent at the location of each bone screw to correctly position the rod for normal or desired curvature of the spinal column. Because the rod is bent, it does not flatly engage the bottom of the groove or U-shaped channel in the head of the bone screw, but tends to be raised from the bottom of the channel at one or both ends. Thus, when a conventional fastener is installed, an outer periphery of the lower end of the fastener most likely engages parts of the rod that are not set snugly against the floor that forms the bone screw channel. After installation, when the patient's back is bent during movement activities, the rod may flex slightly relative to the bone screw. Over time, such flexure may allow the rod to move, either translationally or rotationally, causing the fastener to work loose. Consequently, it is desirable to develop fasteners particularly configured for secure engagement to a spinal implant rod that resist forces placed upon the fastener that tend to loosen the fastener from the bone screw head due to rod flexure caused by patient movement.